A switch-mode power converter (also referred to as a “power converter”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration.
A power converter frequently employs a boost switching regulator topology (also referred to as a “boost regulator” or a “boost topology”) to convert the input voltage waveform into the specified output voltage waveform at a higher voltage level. A boost regulator is commonly used in powering applications that are coupled to an ac mains wherein a rectified input voltage is to be boosted by the boost regulator to produce a dc output voltage higher than the peak value of the ac input voltage or input line voltage supplied thereto.
A conventional single-switch boost regulator is often a configuration of choice in single-phase applications such as line conditioners because the regulator can process an ac input current or input line current with a high power factor. Power factor is a measure of the real power drawn from the mains in watts as a proportion of the apparent volt-amperes supplied. The apparent volt-amperes is the vector sum of the real and reactive power. Unity power factor, therefore, means that the apparent volt-amperes is equal in magnitude to the real power or the reactive power is zero. In ac mains applications where the input line voltage is ideally sinusoidal, achieving unity power factor requires the input line current drawn by the power converter to be controlled to be sinusoidal and aligned in phase with the input line voltage. In these applications, the boost regulator processes the ac input voltage (e.g., 90 volts to 265 volts root mean square (“rms”)) and produces a dc output voltage (e.g., 400 volts), while drawing a substantially sinusoidal input line current aligned with the waveform of the ac input voltage. At a high line voltage of 265 volts, the peak line voltage is roughly 375 volts. Thus, a dc output voltage of 400 volts provides a modest margin for the output voltage to be above the peak input line voltage.
Typically, the controller for a power converter employing a boost topology measures an output characteristic (e.g., the output voltage or the output voltage plus a scaled value of the output current) representing an operating condition of the power converter, and based thereon modifies a duty cycle of a power switch or power switches (also referred to as “switch(es)”) of the power converter to regulate the output characteristic. The duty cycle is a ratio represented by a conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50 percent). The controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to regulate the output characteristic at a desired value. In an exemplary application, power converters have the capability to convert an unregulated ac input voltage such as 120 volts rms to a regulated dc output voltage (e.g., 400 volts dc) to power a load.
An important consideration for the design of a power converter is the efficiency (also referred to as “operating efficiency”) of the power converter in a particular application, and under various operating conditions. The efficiency of a power converter is the ratio of its output power to the input power thereof. An exemplary efficiency of a power converter employing a boost topology while delivering a substantial portion of the rated output power to a load is typically 95 to 97 percent.
Operating efficiency is an important quality indicator for a power converter because of the broad impact efficiency has on equipment reliability and size, operating expense, and corresponding effects on the load equipment powered thereby. Thus, system considerations of achieving high operating efficiency have an immediate effect on the applicability of a particular power converter design, and the associated price the power converter can command in the marketplace.
Numerous prior art attempts have been made to optimize the operating efficiency of a power converter. Many attempts have focused on selection of proper components to provide the maximum operating efficiency for typical operating conditions. In general, a designer focuses on selecting switches with minimal conduction losses and low switching losses at the expected switching frequency. For example, a designer may select a switch formed of a compound semiconductor such as gallium arsenide or silicon carbide to provide low switching losses at higher switching frequencies, such as several hundred kilohertz or higher. Alternatively, a designer may select a switch formed of silicon to take advantage of the low conduction loss and low cost, which may be more relevant at lower switching frequencies, such as 100 kilohertz and lower.
One of the significant circuit elements contributing to power losses in a boost regulator is a boost inductor. Typically, a boost inductor is formed with multiple turns of a copper winding (also referred to as a “winding”) wound around a magnetic core material such as a soft ferrite or powdered iron. Although a powdered iron core can provide low losses at high switching frequencies, it is generally limited by low permeability and its strong dependence on applied field. Soft ferrite cores, on the other hand, can provide higher permeability levels, but are limited in saturation flux density. In addition, inductors formed with multiple layers of copper windings around the magnetic core sustain substantial losses in the windings due to skin and proximity effects, particularly in windings formed of multiple layers. The presence of the boost inductor in boost regulators provides a significant contribution to power converter losses, which raises a challenge to the circuit designers to find a suitable design strategy for the boost inductor.
Examples of power converters employing a boost topology are described in a paper by L. Balogh, et al., entitled “Power Factor Correction With Interleaved Boost Converters in Continuous-Inductor-Current Mode,” IEEE Proceedings of APEC, pp. 168-174, 1993, and in a paper by B. Miwa, et al., entitled “High Efficiency Power Factor Correction Using Interleaving Techniques,” IEEE Proceedings of APEC, pp. 557-568, 1992, both of which are incorporated herein by reference. These papers describe the benefits of using interleaved boost regulators such as reduced volume of magnetic devices to achieve a given level of input ripple current, and to focus on analytical techniques and benefits of the circuit architecture. The aforementioned designs, however, do not employ integrated magnetic devices.
An example of an integrated magnetic device of the prior art for a power converter employing an interleaved boost regulator is given in a paper by Po-Wa Lee, et al. (“Lee”), entitled “Steady-State Analysis of an Interleaved Boost Converter with Coupled Inductors,” IEEE Transactions on Industrial Electronics, Vol. 47, No. 4, pp. 787-795, August 2000, which is incorporated herein by reference. In this paper, Lee describes a power converter employing a boost regulator with coupled boost inductors, but does not include a common winding formed around a common leg of the magnetic core of the coupled boost inductor to provide further reduction in the level of input ripple current.
A further example of the prior art to provide high power converter efficiency is described by M. Rajeev in a paper entitled “An Input Current Shaper with Boost and Flyback Converter Using Integrated Magnetics,” Power Electronics and Drive Systems, The Fifth International Conference on Power Electronics and Drive Systems 2003, Vol. 1, pp. 327-331, 17-20 Nov. 2003, which is incorporated herein by reference. Rajeev describes a power converter employing a single-switch boost regulator using integrated magnetic devices that incorporates three windings. One winding is an independent boost inductor, and the other two windings are the primary and secondary windings of an isolating power transformer (also referred to as a “power transformer” or a “transformer”). There is no substantial magnetic coupling between the boost inductor and the windings of the transformer. While Rajeev achieves some reduction in component volume by forming an integrated magnetic device, the circuit arrangement does not produce a higher ripple frequency for the input and output currents, and does not achieve significant reduction in conduction losses in the windings.
Another example of the prior art to provide high power converter efficiency using an integrated magnetic device is described by I. D. Jitaru, et al. (“Jitaru”), in a paper entitled “Quasi-Integrated Magnetic, An Avenue for Higher Power Density and Efficiency in Power Converters,” Twelfth Annual Applied Power Electronics Conference and Exposition, Vol. 1, pp. 395-402, 23-27 Feb. 1997, which is incorporated herein by reference. Jitaru recognizes that power dissipation in a magnetic device, particularly a magnetic device formed with a soft ferrite, limits the high-frequency flux changes to a level substantially lower than the saturation flux density accessible at low frequencies. Thus, a circuit that can beneficially store energy in the magnetic core at a sufficiently low frequency can advantageously utilize core capability that is otherwise wasted. Jitaru applies this concept to an active clamp dc-to-dc power converter, using either a tapped transformer secondary winding or a current doubler output circuit configuration. While Jitaru is able to provide reduction in volume of magnetic devices, the technique is not applicable to a power converter constructed with a plurality of interleaved boost regulators.
Thus, attempts have been made in the prior art to reduce the volume of magnetic devices for power converters employing a boost topology, and to provide interleaving of boost regulators to increase the effective ripple frequency of high-frequency currents fed back to the input ac mains. Nonetheless, considering the limitations as described above, a coupled inductor for a power converter is presently not available for the more severe applications that lie ahead that depend on achieving high operating efficiency therefor. In accordance therewith, it would be beneficial to provide a coupled inductor formed with, for example, an integrated magnetic device for a power converter that adaptively increases power conversion efficiency, including the considerations of the aforementioned limitations.